Recently, apparatuses having a liquid crystal display (LCD) or other display device and an optical touch panel (an infrared matrix touch panel) mounted on the front surface of the display device (referred to as an interactive machine hereinafter for the convenience of explanation), such as automated teller machines (ATMs) and ticket vending machines, installed in various places including outdoor areas have been increasing.
FIG. 1 is a plan view of a typical optical touch panel 90, FIG. 2 is a plan view of the optical touch panel 90 with an infrared-transparent bezel 107 and a light blocking cover 109 removed, and FIG. 3 is a cross-sectional view of the optical touch panel 90 taken along the line A1-A1 in FIG. 1. Referring to these drawings, the optical touch panel 90 will be described. Note that FIGS. 1 to 3 do not show the interactive machine provided with the optical touch panel 90. Note also that FIGS. 1 to 3 do not show a component that supports substrates 101x, 101y, 91x and 91y, a component to which the light blocking cover 109 is attached, and the like. In the following description, the substrate is referred to as a printed wiring board.
The optical touch panel 90 comprises a rectangular transparent plate (referred to as a transparent plate hereinafter) 105, the infrared-transparent bezel 107 having a property of transmitting infrared light, the light blocking cover 109 that blocks ambient light 21, a plurality of light-emitting devices 103x, a plurality of light-emitting devices 103y, a plurality of light-receiving devices 130x, the number of light-receiving devices 130x being equal to the number of light-emitting devices 103x, a plurality of light-receiving devices 130y, the number of light-receiving devices 130y being equal to the number of light-emitting devices 103y, the two printed wiring boards 101x and 101y, and the two printed wiring boards 91x and 91y. 
Specific examples of the light-emitting devices 103x and 103y include an infrared light-emitting diode (LED). Specific examples of the light-receiving devices 130x and 130y include an infrared phototransistor. The transparent plate 105 is made of a synthetic resin having high transparency (such as poly(methyl methacrylate)) or hardened glass, for example. The infrared-transparent bezel 107 is made of a synthetic resin having infrared transparency (such as methacrylate resin), for example. The light blocking cover 109 is made of a synthetic resin having a light blocking effect, for example. Examples of the ambient light 21 include sunlight and light from an incandescent lamp including infrared light.
As shown in FIG. 2, the plurality of light-emitting devices 103x and 103y and the plurality of light-receiving devices 130x and 130y are arranged to surround the transparent plate 105.
More specifically, the plurality of light-emitting devices 103x are arranged in a row along one of a pair of opposite sides of the transparent plate 105 with the light-emitting surface of each light-emitting device 103x facing to the transparent plate 105. The plurality of light-receiving devices 130x are arranged in a row along the other of the pair of opposite sides of the transparent plate 105 with the light-receiving surface of each light-receiving device 130x facing to the transparent plate 105. In the following description, an axis along which the pair of opposite sides extends is referred to as a Y axis.
Similarly, the plurality of light-emitting devices 103y are arranged in a row along one of the other pair of opposite sides of the transparent plate 105 with the light-emitting surface of each light-emitting device 103y facing to the transparent plate 105. The plurality of light-receiving devices 130y are arranged in a row along the other of the other pair of opposite sides of the transparent plate 105 with the light-receiving surface of each light-receiving device 130y facing to the transparent plate 105. In the following description, an axis along which the other pair of opposite sides extends is referred to as an X axis.
In a configuration in which all the light-emitting devices 103x and 103y and all the light-receiving devices 130x and 130y are arranged around the transparent plate 105, the light-emitting surface of each light-emitting device 103x is opposite to the light-receiving surface of the associated light-receiving device 130x, and the light-emitting surface of each light-emitting device 103y is opposite to the light-receiving surface of the associated light-receiving device 130y. Therefore, in the optical touch panel 90, as shown in FIG. 2, each pair of one light-emitting device 103x and one light-receiving device 130x disposed opposite to each other forms a light path 23x parallel to the X axis, and each pair of one light-emitting device 103y and one light-receiving device 130y disposed opposite to each other forms a light path 23y parallel to the Y axis.
Each light-emitting device 103x has narrow directivity so that the light-emitting device 103x is not optically coupled with the other light-receiving devices than the light-receiving device 130x associated therewith. Similarly, each light-emitting device 103y has narrow directivity so that the light-emitting device 103y is not optically coupled with the other light-receiving devices than the light-receiving device 130y associated therewith.
Assuming that the number of pairs of one light-emitting device 103x and one light-receiving device 130x associated therewith is M, M light paths 23x parallel to the X axis are formed. Assuming that the number of pairs of one light-emitting device 103y and one light-receiving device 130y associated therewith is N, N light paths 23y parallel to the Y axis are formed. Since each light path 23x and each light path 23y in the optical touch panel 90 are perpendicular to each other, the light paths 23x and 23y form a mesh (referred to as an optical mesh hereinafter) when viewed from the front of the transparent plate 105 (see FIG. 2).
In practice, the light-emitting devices 103x are fixed to the printed wiring board 101x, and the light-emitting devices 103y are fixed to the printed wiring board 101y. Similarly, the light-receiving devices 130x are fixed to the printed wiring board 91x, and the light-receiving devices 130y are fixed to the printed wiring board 91y. Typically, a multilayer structure of wiring to drive and control the light-emitting devices 103x is formed on the printed wiring board 101x, a multilayer structure of wiring to drive and control the light-emitting devices 103y is formed on the printed wiring board 101y, a multilayer structure of wiring to drive and control the light-receiving devices 130x is formed on the printed wiring board 91x, and a multilayer structure of wiring to drive and control the light-receiving devices 130y is formed on the printed wiring board 91y. Note that illustration of a control unit, a power supply and the like connected to the printed wiring boards 101x, 101y, 91x and 91y is omitted.
The printed wiring boards 101x, 101y, 91x and 91y are positioned around the transparent plate 105 in such a manner that the light paths 23x and 23y are generally several millimeters apart from one surface (referred to as a touchable plane hereinafter) 105a of the transparent plate 105. The surface of the transparent plate 105 opposite to the touchable plane 105a faces an LCD 25.
As shown in FIGS. 1 and 3, the infrared-transparent bezel 107 has the outer shape of a generally flat rectangular frame. An inner peripheral part 107a of the infrared-transparent bezel 107 has the shape of a flange inclined toward the center of the infrared-transparent bezel 107. The infrared-transparent bezel 107 is fixed to the transparent plate 105 with the inner peripheral edge face of the infrared-transparent bezel 107 being in contact with the periphery of the touchable plane 105a of the transparent plate 105. Thus, the optical touch panel 90 is configured so that the infrared-transparent bezel 107 covers the printed wiring boards 101x, 101y, 91x and 91y like a roof and the inner peripheral part 107a is positioned to interfere with the light paths 23x and 23y. Even with such a configuration, however, the inner peripheral part 107a does not optically block the light paths 23x and 23y because the infrared-transparent bezel 107 has a property of transmitting infrared light.
Since the infrared-transparent bezel 107 has a property of transmitting infrared light, if a light-receiving device 130x or 130y receives infrared light from a source other than the associated light-emitting device 103x or 103y, the optical touch panel 90 malfunctions. To avoid this, in the optical touch panel 90, the light blocking cover 109 shaped not to interfere with formation of the optical mesh and having a size enough to provide a light blocking function is fixed to the back surface of the infrared-transparent bezel 107 to shield the light-receiving devices 130x and 130y from unwanted light (see FIGS. 1 and 3). More specifically, the light blocking cover 109 is shaped and sized not to shield an inner peripheral edge part 107c (a part of the inner peripheral part 107a close to the inner peripheral edge face) and a part that is not exposed to the ambient light 21 but to shield the light-receiving devices 130x and 130y from unwanted light. Since infrared light along the light paths 23x and 23y transmits through the inner peripheral edge part 107c of the infrared-transparent bezel 107, the light blocking cover 109 has no effect on the optical mesh. Note that, in this example, the light blocking cover 109 also shields the light-emitting devices 103x and 103y from unwanted light in such a manner that the light blocking cover 109 does not affect the light paths 23x and 23y. Although FIG. 3 shows the cross section of the optical touch panel 90 taken in the X-axis direction, the cross section of the optical touch panel 90 shown in FIG. 3 can be regarded as the cross section of the optical touch panel 90 taken in the Y-axis direction by replacing the character “x” included in the reference symbols in FIG. 3 with “y” (for this reason, illustration of the cross section of the optical touch panel 90 taken in the Y-axis direction is omitted). Thus, note that the technical descriptions concerning the X-axis direction hold true for the Y-axis direction of the optical touch panel 90.
In the optical touch panel 90, the printed wiring boards 101x, 101y, 91x and 91y are fixed to a supporting component (not shown) in such a manner that the light-emitting devices 103x and 103y and the light-receiving devices 130x and 130y are positioned between the light blocking cover 109 and the printed wiring boards 101x, 101y, 91x and 91y. 
When an obstacle having a certain size, such as a finger of a user and an instrument for manipulation, comes into contact with the touchable plane 105a in the optical touch panel 90 described above, the obstacle typically blocks at least one light path 23x extending in the X-axis direction and at least one light path 23y extending in the Y-axis direction. The two-dimensional position of the obstacle on the transparent plate 105 can be determined by detecting the blocked light paths in the X-axis and Y-axis directions. The information on the two-dimensional position is typically transmitted to the interactive machine.
In the optical touch panel 90, each light path 23x extending in the X-axis direction and each light path 23y extending in the Y-axis direction are perpendicular to each other. However, other than the optical touch panel thus configured, there is an optical touch panel that includes a plurality of light paths that are not perpendicular to each other, such as those disclosed in Japanese Patent Application Laid-Open No. 2003-122504 (referred to as a patent literature 2 hereinafter) and Japanese Patent Application Laid-Open No. 2000-311051 (referred to as a patent literature 3 hereinafter).
As described above, the typical optical touch panel includes at least a transparent plate and a plurality of light-emitting devices and a plurality of light-receiving devices disposed to surround the transparent plate, and a plurality of light paths used to determine the two-dimensional position of the obstacle on the transparent plate, each of which is formed by a pair of a light-emitting device and a light-receiving device, are formed in front of (or above) the transparent plate in such a manner that the light paths form a mesh when viewed from the front of the transparent plate.
Although the typical optical touch panel has the light blocking cover, the ambient light 21 may directly or indirectly reach a light-receiving device 130x or 130y to cause malfunction of the optical touch panel depending on the place where the interactive machine is installed. That is, even when a light path is blocked by an obstacle, the light path may be falsely recognized as not being blocked because of the ambient light 21 reaching the light-receiving device 130x or 130y. In this case, the two-dimensional position of the obstacle on the transparent plate 105 cannot be correctly detected.
An exemplary art to overcome the problem of the optical touch panel is disclosed in Japanese Patent Application Laid-Open No. 2003-040491 (referred to as a patent literature 1 hereinafter). The patent literature 1 discloses an art of reducing the ambient light reaching the light-receiving devices by forming a black serigraph layer on a resist layer of a substrate on which the light-receiving devices are mounted. However, the serigraph layer has a glossy surface, and it is difficult to adequately prevent the reflection of the ambient light from the glossy surface from reaching the light-receiving devices.